1. Field Of The Invention
This invention relates to the preparation of homopolymers and copolymers of ethylene with .alpha.-olefins where the polymerization is carried out to produce polymers of lower mean molecular weight characterized by a broad molecular weight distribution. This invention is particularly directed to a process for preparing a polymer which can be employed in injection molding processes and can also be reprocessed by extrusion.
2. Discussion Of The Prior Art
Various low pressure processes are known for the polymerization of .alpha.-olefins or mixtures thereof. According to Ziegler, catalysts are employed which consist of compounds of the metals of the 4th to the 6th transitional group of the periodic system, preferably titanium compounds and organometallic compounds of the elements of 1st to 3rd main groups of the periodic system, especially aluminum alkyls or alkylaluminum halides. The monomer reaction usually takes place in suspension or in solution, it can however also be conducted in the gas phase.
The products resulting from the process described above are converted by injection molding, blow molding and extrusion into injection molded articles, hollow forms, tubes or films. Each reprocessing procedure and each application area demand products with different physical properties. The mean molecular weight and the molecular weight distribution are the determining factors. This is on account of the fact that a polymolecular product is always obtained on synthesizing macromolecular substances. They consist of macromolecules which are synthesized from the same basic units but differ in their degree of polymerization. The mean molecular weight of a macromolecular substance represents the mean value of the molecular weight for the polymolecular mixture on hand. In order to determine the mean molecular weight several methods are available. For example osmotic measurements, light dispersal measurements, viscosity measurements, and ultracentrifuge experiments can all be used to determine the mean molecular weight.
However, many properties of a macromolecular substance such as toughness, hardness, elasticity, solubility and reprocessing capability, by means of known methods, such as, for example extrusion, are not only determined by the mean molecular weight but depend on the spread of the molecular weights of the macromolecules present in the polymolecular mixture. Polymerizates with a narrow molecular weight distribution are characterized by a high impact strength which is a criterion for the brittleness and toughness of a material. Polymerizates with a wider molecular weight distribution are characterised by improved flow properties and increased stability towards cracking due to stress corrosion.
It is therefore necessary when describing a macromolecular substance not only to define the mean molecular weight but also the distribution of the molecular weight. In order to determine the molecular weight distribution of a macromolecular material, it must be divided up into individual fractions. The molecular weight and amount in each fraction must then be determined. As these methods are involved and time consuming, generally an approximate determination sufficies. The flow properties of polymers can be used to estimate the spread of a molecular weight distribution. For example, the quotient of the melt-indexes of a material, measured at various stresses (MFI.sub.5 or MFI.sub.15 as in DIN ]Deutsche Industrie Norm] No.53735) can serve as a measure of the spread of the molecular weight distribution. The quotient of MIF.sub.15 and MFI.sub.5 is designated the S-value, which is approximately 5 to 20 for polyethylene. Small S-values means narrow, large S-values broad molecular weight distributions.
Polyolefins with a narrow molecular weight distribution e.g., S-values between 6 and 7, and a low molecular weight of approximately 20,000 to 40,000 are especially suitable for injection molding processes. On the other hand, products with a broader molecular weight distribution e.g., S-values between 13 to 17 and a relatively large molecular weight (approximately 80,000 to 200,000) can be readily reprocessed by extrusion.
A polymer with a suitable molecular weight for reprocessing can be obtained by varying the reaction conditions, especially the polymerization temperature by altering the ratio of the catalyst components or by addition of chain-transfer substances to the reaction mixture. Hydrogen is used with preference for the latter purpose. Corresponding processes are, for example, disclosed in DT-PS No. 1 420 390 and DT-AS No. 1,595 666. Products are obtained from the processes described in these publications which have a narrow molecular weight distribution i.e., S-values between 6 and 7 which are ideally suited for injection molding purposes. According to DT-OS No. 1 720 611, in order to attain an ethylene polymer with up to 10 wt.% higher .alpha.-olefins, with a broad molecular weight distribution, the polymerization is conducted in two stages, either in suspension or in the gas phase. The mean molecular weight is regulated by means of hydrogen. According to a favored procedure the composition of the monomer mixture varies in both steps.
In order to regulate the mean molecular weight not only hydrogen is empolyed but also--to a lesser extent--alcohols and/or oxygen. A procedure of this type is described in DT-PT No. 1 210 987 in which catalysts consisting of titanium tetrachloride and dialkylaluminum monochloride are employed. In this case one obtains polymerizates with S-values between 13 and 15.
In practice, the known processes for regulating the mean molecular weight and the molecular weight distribution do not fulfill all requirements. In particular, they do not provide any simple way in which to manufacture polymerizates suitable for the manufacture, by extrusion, of molded articles with a smooth surface and high toughness. The addition of hydrogen, during the polymerization of .alpha.-olefins, can regulate the mean molecular weight within broad limits, however, the products obtained possess a narrow molecular weight distribution with the resultant disadvantages outlined above. On the other hand, the addition of alcohols and/or oxygen makes possible the manufacture of polymerizates with a broad molecular weight distribution. With this regulating system, the mean molecular weight cannot be varied. Multi-step processes for the manufacture of polymerizates with certain mean molecular weights and certain molecular weight distributions are technically involved and can frequently only be conducted discontinuously. Thus, they are not always suitable for an economic manufacture of the polymerizate.
It is, therefore, an object of this invention to overcome the outlined disadvantes of the prior art and to present a process which allows, in particular, the manufacture of polymerizates of ethylene and copolymerizates of ethylene with .alpha.-olefins in which the mean molecular weight can be varied over a wide range, i.e., from approximately 50,000 to approximately 100,000. The S-values for the molecular weight distribution correspond to approximately 7 to approximately 10.